1. Field of the Invention
The invention relates to a clutch mechanism for a vehicle seat and, more particularly, to a clutch mechanism for a vehicle seat, which transmits an occupant's backrest load caused by a rear-end collision of a vehicle.
2. Description of the Related Art
In an existing art, there has been known a headrest for a vehicle seat, of which a portion (support plate) that supports the back of an occupant's head instantaneously pops up toward the back of the occupant's head in the event of a rear-end collision of a vehicle. Here, Japanese Patent Application Publication No. 2005-95237 (JP-A-2005-95237) describes a technique in which the support portion pops up as described above as a load of an occupant seated on a vehicle seat strongly presses a seat back because of momentum in the event of a rear-end collision of a vehicle. In this technique, unless a rear-end collision occurs in the vehicle, an acceleration detecting mechanism installed inside the seat back does not transmit an occupant's backrest load. Therefore, even when the occupant strongly presses the seat back during normal times when no rear-end collision has been occurring in the vehicle, it is possible to prevent the support plate from erroneously popping up.
However, in the above described existing technique, when the occupant is heavy, a mere backrest load of the occupant causes a detection arm to swing by a large amount. On the other hand, when the occupant is light, a load of the occupant pressing the seat back is small even when a rear-end collision occurs in the vehicle, causing the detection arm to swing by a small amount. The support plate pops up when the detection arm swings by a certain amount or more. Therefore, depending on the degree of swing of the detection arm, the acceleration detecting mechanism may not smoothly operate even when a rear-end collision occurs in the vehicle. This may cause a malfunction in popping up of the support plate.